1. Field of the Invention
The present invention relates to a velocity detection apparatus which detects a velocity of a moving object using two detectors.
2. Description of the Related Art
Recently, in order to reduce a color shift in a color image forming apparatus such as a color copying machine or a color laser beam printer, detecting the variation of the conveying velocity of an intermediate transfer belt (a moving object) with high accuracy is required. As a method of detecting the velocity of the moving object with high accuracy, a method of detecting an interval velocity is known. The method of detecting the interval velocity is a method of measuring a transit time of an identifiable feature point on the moving object to detect the velocity of the moving object. Japanese Patent Laid-Open No. 2008-82820 discloses a method of forming two detectors with high accuracy.
However, when the velocity of the moving object is detected by the method of detecting the interval velocity, an averaged velocity in the frequency range in a distance (an interval length) between the detectors is detected. Therefore, only the velocity variation component equal to or less than a certain frequency range which corresponds to the interval length can be detected. Accordingly, when the velocity in a comparatively high frequency range such as a velocity variation component caused by the decentering of a drive roller or the reduction gear decentering of a drive motor is measured, the interval length needs to be shortened.
On the other hand, two detectors (a first detector and a second detector) are disposed on the same substrate with high accuracy using a well-known semiconductor substrate manufacturing process or the like to be capable of shortening the interval length. However, an image detected by the first detector is obtained by overlapping an image that is reflected to be generated by a first light source illuminating a mark on a moving object and an image that is reflected to be generated by a second light source illuminating the mark on the moving object. The same is applied to the second detector. Therefore, a detected signal is distorted by the generation of the crosstalk of the image due to the plurality of light sources and the velocity detection accuracy is reduced.